This invention concerns large air-moving fans rotating in a shroud and provides means for reducing noise level of such fans by several decibels.
Large fans having diameters ranging from about one to seven meters or more are commonly used for moving air through heat exchangers, cooling towers or the like. A typical fan may have a diameter of three meters and from two to eighteen airfoil shaped blades. For light weight and economy, such blades are typically fabricated from thin aluminum alloy sheet. The sheet metal is bent to provide a rounded leading edge, and the rounded upper and lower surfaces of the blade converge toward the trailing edge where they are riveted together. The chord line of the airfoil at the tip of the blade may be in the range of from about 15 to 40 cm. The maximum thickness of the airfoil blade may be in the range of from about 1 to 8 cm.
The blades are typically mounted on a rotatable hub by a clevis. In some manufacturer's fans, the blades are rigidly fixed to the hub in a plane normal to the axis of the fan. In other manufacturer's fans, a resilient member is employed at the mounting so that the blades can "droop" out of such a plane as a result of pressure differences between the two faces of the blade. When the fan is operating, the higher air pressure on the downstream face of the blades tends to force the blades toward the lower pressure face and the blades may droop appreciably away from a plane perpendicular to the axis. In other words, although the blades extend generally radially outwardly from the hub they are not in a plane perpendicular to the axis of the fan. This invention is applicable for either type of fan.
Such large air-moving fans operate within a circumferentially extending shroud. Such shrouds are very often not quite circular and may not be exactly concentric with the axis of the hub. When a fan is installed, the blades and/or shroud are adjusted so that the blades clear the inside of the shroud by one or two millimeters at the closest approach, however, the blades may be 20 to 25 millimeters away from the shroud at the widest gap. Typically, the average gap between the tips of the blades and the shroud is in the order of one centimeter. A small gap is desirable since efficiency decreases as the gap width increases but because of difficulties in maintaining concentricity and a circular shroud, an appreciable average gap is commonly present.
It is generally desirable to operate such fans at relatively high rotation rates to move a large volume of air. However, the noise level from a rapidly rotating large fan may be intolerably high. Noise tends to increase with the fourth power of the rotational velocity. It may therefore be desirable to employ a fan with a relatively larger number of blades so that it can be operated at a lower speed for a given volume of air.
A factor of concern in selecting a fan for a given application is the noise generated by the fan. Blades may be added to the fan so that it can run at lower speed. Noise level also increases with the number of blades, but not at such a rapid rate. However, adding blades also adds cost. Users generally want to rotate a fan at a high velocity to obtain a high airflow. Sound attenuating systems for large fans are quite costly and a low noise fan may avoid need for such systems. Thus, it would be desirable to reduce the noise level from the blades so that a fan could be run at higher rotational speed without objectional noise levels.
It has been found that an appreciable portion of the noise from a large fan is generated as a result of passage of the tip of the blade adjacent to the inside wall of the shroud. The reason for the noise generation in this region is not completely understood, however, certain remedial measures as provided in practice of this invention can reduce the noise level 5 or 6 db. In other words, the noise power may be reduced by a factor of up to four times.